Optical modulator exciting circuit

ABSTRACT

A circuit for exciting an optical modulator, in accordance with the present invention, includes an optical modulator for modulating an optical signal, a first strip line electrically connected to the optical modulator and directing a modulation RF signal to the optical modulator, and a second strip line electrically connected to the first strip line through the optical modulator. The first strip line includes a first electrical device and has a first characteristic impedance, the second strip line includes a second electrical device and has a second characteristic impedance, the first characteristic impedance is equal to a characteristic impedance of a path through which the modulation RF signal is input into the first strip line, and a parallel-combined impedance of the first and second electrical devices is equal to the characteristic impedance of the path.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a circuit for launching a modulation signal toan optical modulator which, modulates an optical signal, an opticalmodulator module including a circuit for launching a modulation signalto an optical modulator, and a method of launching a modulation signalto an optical modulator which modulates an optical signal.

2. Description of the Related Art

With significant increase in demand for broad-band multi-mediacommunication services such as Internet, it is now required to developan optical-fiber communication system having a greater capacity andbeing able to accomplish higher performance. In addition, with increasein a scale of an optical-fiber communication system, the number ofoptical communication modules used in an optical-fiber communicationsystem is increasing.

For the reasons mentioned above, a cost and a load of assembly, as wellas a size, of an optical communication module used in an optical-fibercommunication system are not ignorable in an optical-fiber communicationsystem. Thus, it is now quite important to fabricate an opticalcommunication module in a smaller size, under higher integration offunctions, and in smaller costs.

As one of solutions of fabricating an optical-fiber communication systemin a smaller size and reducing the number of parts of an optical-fibercommunication system, time-multiplicity of data may be increased toincrease data-transmission capacity per one wavelength channel. In orderto accomplish the solution, an optical communication device associatedwith high-speed modulation is presently researched and developed.

On the other hand, if data-transmission data per one wavelength channelis increased, wavelength dispersion inherent to an optical fiber pathexerts non-ignorable influence on an optical waveform found after longdistance transmission. This is because when optical intensity modulationis applied to a light source device, phase modulation (or frequencymodulation), though it is quite small, is also applied to the lightsource device. Such phenomenon is called “wavelength chirping”, andexerts serious influence on long-distance transmission characteristic ifa transmission rate is over 2.5 Gb/s per a channel. Hence, an externalmodulation system in which wavelength chirping is small is mainlyapplied to an optical-fiber communication system acting as a trunk line.Further developed now for an external modulation system are a singleoptical-intensity modulator making use of electroabsorption effect ofchemical compound semiconductor, a light source including a singleoptical-intensity modulator and a light source device such as a DFBlaser both integrated together in monolithic.

Presently, an optical-fiber communication system having a transmissionrate in the range of 2.5 Gb/s and 10 Gb/s per a channel has been alreadyin practice use. There are now developed a ultra-high-speedelectroabsorption type optical modulator, an integrated light sourceused for the optical modulator, and a pigtailed module including them,in order to accomplish a high transmission rate over 40 Gb/s per achannel.

Such an exciter for launching a modulation signal as illustrated in FIG.7 is often used for integrating an electroabsorption type opticalmodulator and a modulator-integrated light source. FIG. 7 is a schematicof a conventional circuit 100 for launching a modulation signal to anoptical modulator.

The circuit 100 for launching a modulation signal to an opticalmodulator is comprised of an optical modulator 111, a first stripline112, a second stripline 113 having a terminator 114, a first bondingwire 115 connecting the optical modulator 111 and the first stripline112 to each other, and a second boding wire 116 connecting the opticalmodulator 111 and the second stripline 113 to each other.

The first and second striplines 112 and 113 are arranged sandwiching theoptical modulator 111 therebetween in a line in a directionperpendicular to a direction in which an optical signal 131 propagates.The optical signal 131 is coupled into the optical modulator 111, andmodulated into a second optical signal 312 in the optical modulator 111in accordance with a modulation signal 133 output from the firststripline 112.

The circuit 100 can control modulation bandwidth by changing aninductance of the first and second bonding wires 115 and 116. However,the circuit 100 is accompanied with a serious problem that thereflection S₁₁ (see later mentioned FIG. 5) of the circuit 100 issignificantly increased in a high-frequency band close to a millimeterwave band and above. This is due to the following two reasons.

The first reason is as follows. A susceptance of the optical modulator111 acting as if it is a capacitor, when reverse-biased, is equal tozero (open-circuit) in the vicinity of a direct current. However, thesusceptance increases up to almost the same level as characteristicadmittance of the first and second striplines 112 and 113 in a highband, and hence, turned into a low impedance (a load almost equal toshort-circuit).

The second reason is as follows. A load reflecting the second bodingwire 116 and subsequent parts becomes a high impedance in the range of aresonance frequency, defined based on both an inductance of the secondbonding wire 116 connecting the optical modulator 111 and the secondstripline 113 each other, and a capacitance of an optical absorptionlayer of the optical modulator 111, and a high frequency. Accordingly,the terminator 114 is unlikely to effectively work.

Since an absolute value of the reflection may exceed −10 dB (10% of amodulation RF signal input power), it would be unavoidable that theoptical modulator 111 would have much burden, or unnecessary resonancemight be found in modulation frequency characteristics.

As a solution to those problems, arrangement of a fixed attenuator in astage prior to a module for damping a reflection wave to a certain levelor below is considered to be the easiest. However, it is difficult toaccomplish a broad band and high output both required in such a drivercircuit especially for a 40 Gb/s band. Thus, this solution cannot beadopted, because the solution causes a burden with a driver circuit.

For the purpose of improving such impedance mismatching, a strip-linebased stub circuit having open or short-circuited ends are often used.However, such stubs can compensate such an impedance mismatch of a loadat only a particular frequency, so that it is not suitable for achievingbroad-band matching in the range of direct current to millimeter wave.In addition, a stub may not be suitable as such matching circuit whichshould be deployed in a package having limited dimensions like theoptical modulator module from the viewpoints of module design andassembly.

However, since there are no effective solutions other than theabove-mentioned solutions, an optical modulator module and amodulator-integrated light source, both of which are still used, thoughthe above-mentioned problems remain unsolved and intensive reflection isnot solved.

For instance, Japanese Patent Application Publication No. 2001-209017has suggested a photoelectric-transfer semiconductor device whichcarries out impedance-matching in a broad band to provide a highphotoelectric frequency. The suggested photoelectric-transfersemiconductor device is comprised of a semiconductor element, ahigh-frequency electric signal circuit, a circuit for matchingresistors, and a circuit for matching capacitors. The semiconductorelement carries out photoelectric transfer. The high-frequency electricsignal circuit has ends located in the vicinity of the semiconductorelement. Among the ends, an end located closest to an electric-signalterminal of the semiconductor element is electrically connected to theelectric-signal terminal through an electrical conductor. The circuitfor matching resistors is electrically connected at one end to theelectric-signal terminal of the semiconductor element through anelectrical conductor, and is grounded at the other end. The circuit formatching capacitors is electrically connected to an end of thehigh-frequency electric signal circuit, and has such an impedance thatan impedance at the side of the semiconductor element when viewed fromthe end is equal to a standardized impedance of the circuit for matchingresistors.

Japanese Patent Application Publication No. 2001-154161 has suggested asemiconductor device which allows photo carriers having been generatedin the device to sweep out therefrom. The suggested semiconductor deviceis comprised of a semiconductor element, and a short-circuit element.The semiconductor element is comprised of a semiconductor layerreceiving an optical high-frequency signal having a particularfrequency, and generating photo carriers, and an output electrode whichoutputs the photo carriers as high-frequency. The short-circuit elementis electrically connected to the output electrode, and keeps the outputelectrode grounded for the high-frequency.

Japanese Patent Application Publication No. 2000-19473 has suggested astructure of an optical modulator module for using a microstrip linehaving a small space in which a transmission line is to be fabricated.The suggested structure is comprised of an optical device, a carrier, anoptical fiber, a high-frequency terminal, a thermoelectric cooler, adielectric substrate, and a package. The carrier has electricalconductivity, and the optical device is mounted on the carrier. Anoptical signal is input into and output from the optical device by theoptical fibers. The high-frequency terminal provides an electrichigh-frequency signal. The thermoelectric cooler keeps the opticaldevice at a constant temperature. A microstrip line is formed on thedielectric substrate. The package holds the above-mentioned parts in it.The package has the high-frequency terminal and a coplanar waveguide.The dielectric substrate is mounted on the carrier. The carrier isexposed at an end thereof closer to the high-frequency terminal. Theexposed portion of the carrier, a ground of the coplanar waveguide, themicrostrip line, and a signal area of the coplanar waveguide areconnected to one another with the bonding wires.

However, the above-mentioned problem of the reflection remains unsolvedeven in the above-mentioned photoelectric-transfer semiconductor deviceand the structure of an optical modulator module.

Accordingly, it is an object of the present invention to provide acircuit for launching a modulation signal to an optical modulator, thecircuit being capable of suppressing significant increase in reflectionparticularly in a high-frequency band in which a maximum frequency of amodulation RF signal reaches a millimeter wave band, when an opticalmodulator and an optical modulator module on which an optical modulatoris integrated are modulated at a high-speed.

It is also an object of the present invention to provide a circuit forlaunching a modulation signal to an optical modulator, which is capableof suppressing significant increase in the above-mentioned reflectionwithout deteriorating a modulation frequency band.

It is further an object of the present invention to provide a circuitfor launching a modulation signal to an optical modulator, which iscapable of suppressing significant increase in the above-mentionedreflection without necessity of changing circuit elements, parts and amethod of fabricating them.

It is further an object of the present invention to provide a circuitfor launching a modulation signal to an optical modulator which is mostsuitable for accomplishing a broader band of an optical modulator moduleincluding an optical modulator and a device in which the opticalmodulator is integrated, driving the optical modulator module at a lowervoltage, fabricating the optical modulator module at lower costs, andaccomplishing mass production of the optical modulator module.

It is further an object of the present invention to provide an opticalmodulator module including the above-mentioned circuit for launching amodulation signal to an optical modulator, and a method of launching amodulation signal to an optical modulator, which provides the sameadvantages as the advantages provided by the above-mentioned circuit forlaunching a modulation signal to an optical modulator.

SUMMARY OF THE INVENTION

In order to accomplish the above-mentioned object, the present inventionprovides a circuit for launching a modulation signal to an opticalmodulator, including an optical modulator for modulating an opticalsignal, a first stripline electrically connected to the opticalmodulator and directing a modulation RF signal to the optical modulator,and a second stripline electrically connected to the first striplinethrough the optical modulator, characterized in that the first striplineincludes a first electrical device and has a first characteristicimpedance, the second stripline includes a second electrical device andhas a second characteristic impedance, the first characteristicimpedance is equal to a characteristic impedance of a path through whichthe modulation RF signal is input into the first stripline, and aparallel-combined impedance of the first and second electrical devicesis equal to the characteristic impedance of the path.

The second electrical device may be comprised of at least two electricaldevices, in which case, it is preferable that the at least twoelectrical devices are arranged at different locations from each otherin a length-wise direction of the second stripline.

As an alternative, the at least two electrical devices may be arrangedat opposite ends of the second stripline.

At least one of the first and second electrical devices may be comprisedof a resistor, for instance.

The resistor may be comprised of a thin-film resistor formed on anelectrical conductor of the second stripline.

It is preferable that the second stripline has an electrical lengthequal to or smaller than a quarter of a wavelength associated with amaximum frequency of the modulation RF signal.

The second characteristic impedance may be designed to be equal to thefirst characteristic impedance. However, it is preferable that thesecond characteristic impedance is different from the firstcharacteristic impedance.

The first electrical device may be designed to have an impedance equalto the first characteristic impedance. However, it is preferable thatthe first electrical device has an impedance different from the firstcharacteristic impedance.

The present invention further provides an optical modulator moduleincluding a high-frequency input section for receiving a modulation RFsignal by which an optical signal is modulated, an optical input sectionfor receiving a first optical signal, a circuit for launching amodulation signal to an optical modulator, electrically connected to thehigh-frequency input section and optically connected to the opticalinput section, the circuit modulating the first optical signal into asecond optical signal in accordance with the modulation RF signal, thecircuit being comprised of a circuit defined in any one of claims 1 to8, and an optical output section optically connected to the circuit andoutputting the second optical signal.

The optical input section may be comprised of an optical input terminalthrough which an optical signal is input and output, and a first lens,in which case, the optical input terminal is connected to a firstoptical fiber, receives the first optical signal through the firstoptical fiber, and outputs the first optical signal to the first lens,and the first lens receives the first optical signal and outputs thefirst optical signal to the optical modulator of the circuit.

The optical output section may be comprised of a second lens, and anoptical output terminal through which an optical signal is input andoutput, in which case, the second lens receives the second opticalsignal from the optical modulator of the circuit, and outputs the secondoptical signal to the optical output terminal, and the optical outputterminal is connected to a second optical fiber, receives the secondoptical signal through the second lens, and outputs the second opticalsignal to the second optical fiber.

The present invention further provides a method of launching amodulation signal to an optical modulator, including outputting amodulation RF signal to an optical modulator through a first striplineincluding a first electrical device and having a first characteristicimpedance, modulating a first optical signal into a second opticalsignal in the optical modulator in accordance with the modulation RFsignal, and outputting the modulation RF signal through the opticalmodulator to a second stripline including a second electrical device andhaving a second characteristic impedance, wherein the firstcharacteristic impedance is equal to a characteristic impedance of apath through which the modulation RF signal is input into the firststripline, and a parallel-combined impedance of the first and secondelectrical devices is equal to the characteristic impedance of the path.

The circuit for launching a modulation signal to an optical modulator inaccordance with the present invention provides super characteristics ina broad-band characteristic and a low-reflection characteristic when anoptical modulator is excited in accordance with a modulation RF signal.

That is, the circuit for launching a modulation signal to an opticalmodulator in accordance with the present invention suppresses thebehavior of an optical modulator as a capacitive load as much aspossible by optimizing a stripline in an optical modulator moduleincluding an optical modulator and an optical element in which theoptical modulator is integrated. In addition, the circuit in accordancewith the present invention minimizes an influence exerted on amodulation frequency band, and lowers reflection for the circuit down toa target level.

Specifically, in the circuit for launching a modulation signal to anoptical modulator in accordance with the present invention, terminatorresistors of an existing optical modulator module are arranged in thecircuit in dispersion. As a result, it would be possible to lower thereflection characteristic of the circuit for launching a modulationsignal to an optical modulator, to a practically ignorable level. It ispossible to do so by making use of a conventional apparatus forfabricating the circuit, a conventional method of doing the same, and aconventional process of assembling a module without any additionalcircuit elements and parts.

The arrangement of terminator resistors in dispersion in dispersion canbe summarized as follows.

(1) A susceptance of an optical modulator element is matched with a loadhaving a sign opposite to the sign of the susceptance.

(2) As a load used for the matching, the second stripline is designed tohave terminator resistors at opposite ends thereof A matching circuit iscomprised of the second stripline having terminator resistors atopposite ends thereof, and an optical modulator element electricallyconnected in parallel to the second stripline.

A structure of forming terminator resistors at opposite ends of thesecond stripline constitutes a stub having terminated opposite ends,unlike a conventional stub. The reason for not using a conventional openor short-circuited stub as a circuit for matching a micro wave andmillimeter wave band is that since an impedance of the open orshort-circuited stub significantly varies together with a frequency, theopen or short-circuited stub is not suitable for impedance-matching ofan optical modulator element acting as a capacitive load.

It is possible to independently optimize upper and lower limits of animpedance of a requisite matching circuit by varying a ratio between acharacteristic impedance and a terminator resistance in the secondstripline. The terminators formed at opposite ends of the secondstripline acts as a dumping resistor for suppressing unnecessarymultiple reflections occurring between discontinuous sections of a lineformed in a circuit for launching a modulation signal to an opticalmodulator. The terminators attenuate unnecessary reflected waves, andeffectively suppress an influence exerted by unnecessary resonance onmodulation frequency characteristic.

(3) The second stripline is designed to have an electric length equal toor smaller than a quarter of a wavelength of an input modulation RFsignal.

A matching band is almost dependent on a length of the second stripline.It is now assumed that a second stripline has a length equal to orsmaller than a quarter of a wavelength λ of an input modulation RFsignal, wherein primary approximation is established for the wavelengthλ, when Taylor development is applied to an admittance of the secondstripline. Under the assumption, it is possible to consider the secondstripline and the terminator resistors as a matching circuit which showsa tendency that an admittance is reduced (or an impedance is increased)in proportion with a frequency.

The behavior of the second stripline and the terminator resistors cancelthe behavior of a capacitive load such as an optical modulator elementwhich is simply dependent on a frequency (that is, an impedance isreduced). Since the behavior of the second stripline and the terminatorresistors meet with the requirement set forth in (1), and hence, theyare suitable for accomplishing a broader band.

(4) A parallel-combined resistance of the terminators arranged indispersion in the first and second striplines is set equal to acharacteristic impedance of a line through which a modulation RF signalis input.

In a low-frequency band close to a direct current, an impedance of anoptical modulator element is almost indefinite (open-circuit), and aload of a circuit for launching a modulation signal to an opticalmodulator is almost equal to a parallel-combined resistance of theterminators arranged in dispersion. Hence, if the terminators of thefirst and second striplines are selected so that the load is equal to acharacteristic impedance of a line through which a modulation RF signalis input, it would be possible to suppress the reflection of a circuitfor launching a modulation signal to an optical modulator, down to apractically ignorable level.

In general, it is not easy to optimize parameters of circuit elements ofa matching circuit due to dependency of an impedance of an element to bematched, on a frequency. However, the circuit for launching a modulationsignal to an optical modulator in accordance with the present inventionmakes it possible to almost independently optimize upper and lowerlimits of an absolute value of an impedance of a matching circuit byvarying a length of the second stripline for controlling the dependencyof a matching circuit on a frequency, based on the characteristic setforth in (3), and further by controlling a ratio between acharacteristic impedance and resistances of resistors formed at oppositeends in the second stripline.

In other words, since an admittance tends to decrease (or an impedancetends to increase) in proportion to a frequency of a modulation RFsignal in a matching circuit, it would be possible to control increaseand decrease of a range or a band in which a frequency is matched, and adegree at which a range or a band in which a frequency is matched isincreased or decreased.

Furthermore, it is possible to control an upper limit of an absolutevalue of an impedance of a matching circuit by controlling a ratiobetween a characteristic impedance of the second stripline and aresistance of resistors formed at opposite ends of the second stripline.

Similarly, it is possible to control a lower limit of an absolute valueof an impedance of a matching circuit by controlling a ratio between acharacteristic impedance of the second stripline and a resistance ofresistors formed at opposite ends of the second stripline.

In addition, it is possible to optimize the above-mentioned upper andlower limits independently of each other.

The present invention makes it possible to solve a problem that thereflection is significantly increased in a high-frequency band when anoptical modulator and an optical modulator module in which the opticalmodulator is integrated are modulated at a high-speed, and hence,accomplishes reduction in reflection in a broad band in the range of adirect current and a millimeter wave band which are required for anoptical-fiber communication system. Thus, it is possible to reduce aburden exerted on a circuit for driving an optical modulator module, andfurther reduce a harmful influence exerted on a modulation signal,caused by unnecessary resonance.

Furthermore, the present invention does not degrade a modulationfrequency band, and provides superior broad-band and low-reflectionmodulation characteristic.

In fabrication of a circuit for launching a modulation signal to anoptical modulator in accordance with the present invention, a minimumchange, specifically, addition of a thin-film resistor is applied to astripline of a conventional circuit for launching a modulation signal toan optical modulator. That is, conventional process and equipments forfabricating the circuit can be used without any changes except that amask pattern for fabricating a thin-film resistor is partially revised.Thus, it is possible to have an optical modulator module had highperformance without any additional costs, enhancement in performance,enhancement in productivity and reduction in costs can be expected. As aresult, it is possible for an optical-fiber communication system actingas a trunk line to operate at a higher rate and to present highperformances, for construction of a next-generation communicationnetwork.

The above and other objects and advantageous features of the presentinvention will be made apparent from the following description made withreference to the accompanying drawings, in which like referencecharacters designate the same or similar parts throughout the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a circuit for exciting an opticalmodulator in accordance with the first embodiment of the presentinvention.

FIG. 2 is a plan view of an embodiment of an electric-field absorptiontype optical modulator module including the circuit for exciting anoptical modulator in accordance with the first embodiment of the presentinvention.

FIG. 3 is a perspective view of an optical modulator in the circuit forexciting an optical modulator in accordance with the first embodiment ofthe present invention.

FIG. 4 is a perspective view of an example of the circuit for excitingan optical modulator in accordance with the first embodiment of thepresent invention.

FIG. 5 is a graph showing the measurement results of a relation betweena frequency and reflection in a circuit for exciting an opticalmodulator.

FIG. 6 is a graph showing the measurement results of a relation betweena frequency and reflection in a circuit for exciting an opticalmodulator.

FIG. 7 is a perspective view of a conventional circuit for exciting anoptical modulator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A circuit for launching a modulation signal to an optical modulator andan optical modulator module in accordance with the preferred embodimentsof the present invention are explained hereinbelow with reference todrawings.

First, a circuit for launching a modulation signal to an opticalmodulator in accordance with the first embodiment of the presentinvention is explained hereinbelow.

FIG. 1 is a perspective view of the circuit 10 for launching amodulation signal to an optical modulator in accordance with the firstembodiment of the present invention.

The circuit 10 is comprised of an optical modulator 1, a first stripline2, a first terminator 3 formed on the first stripline 2, a secondstripline 4, a second terminator 5 and a third terminator both formed onthe second stripline 4, a first bonding wire 8 through which the opticalmodulator 1 and the first stripline 2 are electrically connected to eachother, and a second bonding wire 9 through which the optical modulator 1and the second stripline 4 are electrically connected to each other.

The optical modulator 1 modulates a received optical signal 31 withrespect to an optical intensity, a frequency and a phase in accordancewith a modulation RF (Radio Frequency) signal 33 (electric field), andoutputs an optical signal 32 as a modulated optical signal.

The first and second striplines 2 and 4 are arranged sandwiching theoptical modulator 1 therebetween in a line perpendicularly to an opticalaxis of the optical modulator. The first and second striplines 2 and 4are electrically connected to each other through the first and secondbonding wires 8 and 9 with the optical modulator 1 being electricallyconnected to the first and second striplines 2 and 4.

In the circuit 10 for launching a modulation signal to an opticalmodulator in accordance with the first embodiment, the first stripline 2has a characteristic impedance Z01 and an effective index of refractionNm1, and the second stripline 4 has a characteristic impedance Z02 andan effective index of refraction Nm2.

The first stripline 2 receives a modulation RF signal 33 used formodulating an optical signal 31 passing through the optical modulator 1,and then, outputs the modulation RF signal 33 to the optical modulator 1through the first bonding wire 8. The characteristic impedance Z01 ofthe first stripline 2 is equal to a characteristic impedance Z0 of aline through which the modulation RF signal 33 is output.

The second stripline 4 receives the modulation RF signal 33 through theoptical modulator 1 and the second bonding wire 9.

In the circuit 10 in accordance with the first embodiment, thecharacteristic impedance Z02 of the first stripline 4 is set differentfrom the characteristic impedance Z01 of the first stripline. This is toensure that the reflection of the modulation RF signal 33 remains in thesecond stripline 4. By allowing the reflection of the modulation RFsignal 33 to remain, it is possible to cause the first, second and thirdterminators 3, 5 and 6 to present their advantages. In the circuit 10 inaccordance with the first embodiment, the second stripline 4 is designedto have a length equal to or smaller than C0/(4Fm×Nm2) wherein Fmindicates an expected maximum frequency Fm of the modulation RF signal33, Nm2 indicates an effective index of refraction of the secondstripline 4, and C0 indicates a luminous flux. This means that anelectrical length of the second stripline 4 is equal to or smaller thana quarter of a wavelength associated with a maximum frequency Fm of themodulation RF signal 33.

The first terminator 3 is arranged at an end of the first stripline 2closer to the optical modulator 1, and defines an electrical devicehaving an impedance of ZL1. In the first embodiment, the impedance ZL1is set different from the characteristic impedance Z01 of the firststripline. On a surface of the first stripline 2 is formed an electricalconductor 2 a extending across opposite ends of the first stripline in alength-wise direction thereof. In the first embodiment, the firstterminator 3 is comprised of a thin-film resistor formed on theelectrical conductor 2 a.

The second and third terminators 5 and 6 are arranged at differentlocations on the second stripline 4 in a length-wise direction thereof,and define electrical devices having impedances ZL2 and ZL3,respectively. In the first embodiment, the second and third terminators5 and 6 are arranged at opposite ends of the second stripline 4 in alength-wise direction thereof. On a surface of the second stripline 4 isformed an electrical conductor 4 a extending across opposite ends of thesecond stripline 4 in a length-wise direction thereof. In the firstembodiment, each of the second and third terminators 5 and 6 iscomprised of a thin-film resistor formed on the electrical conductor 4a.

In the first embodiment, a parallel-combined resistance (impedance) ofthe three terminators (the first terminator 3, the second terminator 5and the third terminator 6) is set equal to the characteristic impedanceZ0 of a line through which the modulation RF signal 33 is output.Accordingly, in the first embodiment, the parallel-combined resistance(impedance) of the three terminators is also equal to the characteristicimpedance Z01 of the first stripline 2.

It is preferable that a sum of resistances of the three terminators, thefirst bonding wire 8 and the second bonding wire 9 is equal to thecharacteristic impedance Z0. If the modulation RF signal 33 had a lowfrequency signal, that is, the modulation RF signal is almost a directcurrent, impedances of the first and second bonding wires 8 and 9 areignorable.

The first bonding wire 8 electrically connects the first stripline 2 andthe optical modulator to each other, and the second bonding wire 9electrically connects the second stripline 4 and the optical modulator 1to each other. In addition, the first and second bonding wires 8 and 9are electrically connected to each other on the optical modulator 1.

An example of an optical modulator module 20 including the circuit 10 inaccordance with the first embodiment is explained hereinbelow withreference to FIG. 2.

The optical modulator module 20 is comprised of a carrier 7, a firstlens unit 21-1, a second lens unit 21-2, a high-frequency connector 22,a package 22, an optical input terminal 24-1, an optical output terminal24-2, a thermal sensor 25, a circuit 27 for launching a modulationsignal to an optical modulator, and a radiator 28.

The circuit 27 has the same structure as the structure of the circuit 10in accordance with the first embodiment, illustrated in FIG. 1.

The carrier 7 is comprised of a base composed of a metal. On the carrier7 and in the optical modulator module 20 are arranged the circuit 27,the first lens unit 21-1, the second lens unit 21-2 and the thermalsensor 25 in a predetermined positional relation.

Each of the first lens unit 21-1 and the second lens unit 21-2 iscomprised of a lens and a lens holder supporting the lens therewith.Lenses of the first lens unit 21-1 and the second lens unit 21-2 arepositioned on an optical axis connecting an optical waveguide of theoptical modulator 1 of the circuit 27 and cores of the first and secondoptical fibers 26-1 and 26-2 to each other. A distance between the firstand second lens units 21-1 and 21-2 and the optical modulator 1 isdetermined such that foci of the lenses are situated on end surfaces ofthe optical modulator 1 (end surfaces of the optical waveguide) throughwhich an optical signal is input and output.

The high-frequency connector 22 electrically connects a transmissionline (not illustrated) of the modulation RF signal 33 and the opticalmodulator module 20 to each other, and outputs the modulation RF signal33 transmitted through the transmission line, to the first stripline 2of the circuit 27.

The carrier 7 is mounted on an electronic cooler mounted on an innerbottom of the package 23 (since the electronic cooler is located belowthe carrier 7, the electronic cooler is not illustrated in FIG. 2).

The optical input terminal 24-1 optically connects the first opticalfiber 26-1 and the optical modulator module 20 to each other, and theoptical output terminal 24-2 optically connects the second optical fiber26-2 and the optical modulator module 20 to each other. The opticalinput terminal 24-1 outputs the optical signal 31 having beentransmitted through the first optical fiber 26-1, to the first lens unit21-1 of the optical modulator module 20, and the optical output terminal24-2 outputs the optical signal 32 having been output through the secondlens unit 21-2 of the optical modulator module 20, to the second opticalfiber 26-2.

The thermal sensor 25 measures a temperature in the vicinity of thecircuit 27.

Bias leads 28 are electrically connected to the thermal sensor 25 andthe electronic cooler.

Hereinbelow is explained an operation of the optical modulator module 20with reference to FIGS. 1 and 2.

The modulation RF signal 33 having been transmitted through a line isoutput to the first stripline 2 of the circuit 27 in the opticalmodulator module 20 through the high-frequency connector 22. Themodulation RF signal 33 having been input into the first stripline 2through an input end of the first stripline is output to the opticalmodulator 1 from the first stripline 2 through the first bonding wire 8.

The optical signal 31 having been transmitted through the first opticalfiber 26-1 is output to the optical modulator module 20 through theoptical input terminal 24-1. The optical signal 31 is received at thefirst lens unit 21-1, and focused onto an end of the optical waveguideof the optical modulator 1 through the lens of the first lens unit 21-1.Then, the optical signal 31 is transmitted towards the other end of theoptical waveguide through the optical waveguide of the optical modulator1.

The optical signal 31 is modulated in accordance with the modulation RFsignal 33 having been input through the first bonding wire 8, whilebeing transmitted through the optical waveguide, and then, turned intothe optical signal 32 as a modulated optical signal. The optical signal32 is transmitted through the other end of the optical waveguide, and isreceived at the second lens unit 21-2. Then, the optical signal 32 isfocused onto an end of the optical output terminal 24-2 through the lensof the second lens unit 21-2, and then, output to the second opticalfiber 26-2 through the optical output terminal 24-2.

After having modulated the optical signal 31, the modulation RF signal33 is transmitted to a distal end of the second stripline 4 through theoptical modulator 1.

When the modulation RF signal 33 is a low-frequency signal close to adirect current, the optical modulator 1 has an almost infinite impedance(open-circuit). In addition, since the first and second bonding wires 8and 9 have ignorable impedances, an input impedance of the circuit 27,viewed from a line through which the modulation RF signal 33 istransmitted, is almost equal to a parallel-combined resistance of thefirst, second and third terminators 3, 5 and 6. Hence, it would bepossible in a low-frequency band to suppress the reflection of thecircuit 27 down to a practically ignorable level, because acharacteristic impedance of a line through which the modulation RFsignal 33 is transmitted is equal to an impedance at an end(parallel-combined resistance=z0).

In contrast, when the modulation RF signal 33 has a high frequencycovering a GHz wave band to a millimeter wave band, an impedance of theoptical modulator 1 acting as a capacitive load is significantlyreduced. The second stripline 4 is designed to have a length equal to orsmaller than λ/4 which allows primary approximation to be establishedfor a frequency of an admittance. Hence, it is possible to consider thesecond stripline 4 as a matching circuit exhibiting a tendency that anadmittance is decreased in proportion with a frequency (that is, animpedance is increased in proportion with a frequency). That is, a sumof impedances of the second bonding wire 9, the second stripline 4, thesecond terminator 5 and the third terminator 6 all arranged downstreamof the optical modulator 1 compensates for the behavior of the opticalmodulator 1, namely, rapid reduction of an impedance of the opticalmodulator 1 (in other words, the sum of impedances rapidly increase),and cancels the dependency of an impedance reflecting the circuit 27 ona frequency.

Thus, the optical modulator module 20 suppresses the reflection of thecircuit 27 down to a practically ignorable level in a broad range of adirect current to a maximum frequency.

Furthermore, if a conventional matching process in which a capacitor isadded to the first stripline 2 is applied to the optical modulatormodule 20 in accordance with the first embodiment, it would be possibleto further enhance the low-reflection characteristic.

EXAMPLE 1

Hereinbelow is explained an example of applying the circuit forlaunching a modulation signal to an optical modulator in accordance withthe first embodiment to an electroabsorption type optical modulatormodule.

FIG. 2 illustrates an electroabsorption type optical modulator module(the optical modulator module 20). Since the optical modulator modulehas been explained above, the parts other than the circuit 27 explainedhereinbelow are not explained.

FIG. 4 is a perspective view of the circuit 27 for launching amodulation signal to an optical modulator, applied to theelectroabsorption type optical modulator module 20.

As illustrated in FIG. 4, the circuit 27 as a part of theelectroabsorption type optical modulator module 20 is comprised of anoptical modulator 11, a first stripline 12, a first electrical conductor12-1 formed on an upper surface of the first stripline 12, a secondelectrical conductor 12-3 (not illustrated) formed on a lower surface ofthe first stripline 12, a first thin film 13-2 formed on upper, side andlower surfaces of the first stripline 12 at an end of the firststripline 12 closer to the optical modulator 11, and electricallyconnecting the first and second electrical conductors 12-1 and 12-3 toeach other, a first terminator 13-1 formed on the first electricalconductor 12-1 and the first thin film 13-2 at an end of the firststripline 12 closer to the optical modulator 11, a second stripline 14,a third electrical conductor 14-1 formed on an upper surface of thesecond stripline 14, a fourth electrical conductor 14-3 (notillustrated) formed on a lower surface of the second stripline 14, asecond thin film 15-2 formed on upper, side and lower surfaces of thesecond stripline 14 at an end of the second stripline 14 closer to theoptical modulator 11, and electrically connecting the third and fourthelectrical conductors 14-1 and 14-3 to each other, a third thin film16-2 formed on upper, side and lower surfaces of the second stripline 14at an end of the second stripline 14 remoter from the optical modulator11, and electrically connecting the third and fourth electricalconductors 14-1 and 14-3 to each other, a second terminator 15-1 formedon the third electrical conductor 14-1 and the second thin film 15-2 atan end of the second stripline 14 closer to the optical modulator 11, athird terminator 16-1 formed on the third electrical conductor 14-1 andthe second thin film 15-2 at an end of the second stripline 14 remoterfrom the optical modulator 11, a first Au ribbon wire 18 electricallyconnecting the optical modulator 11 and the first stripline 12 to eachother, and a second ribbon wire 19 electrically connecting the opticalmodulator 11 and the second stripline 14 to each other.

The optical modulator 11 corresponds to the optical modulator 1 of thecircuit 10 illustrated in FIG. 1. Similarly, the first stripline 12corresponds to the first stripline 2, the first terminator 13-1 to thefirst terminator 3, the second stripline 14 to the second stripline 4,the second terminator 15-1 to the second terminator 5, the thirdterminator 16-1 to the third terminator 6, the first Au ribbon wire 18to the first bonding wire 8, and the second Au ribbon wire 19 to thesecond bonding wire 9.

The first stripline 12 is comprised of a coplanar line comprised of analumina substrate 12-2 having a dielectric constant of 9.95, a thicknessof 250 micrometers, a length of 3.5 mm, and a width of 700 micrometers,and has a characteristic impedance of 50 ohms. The first thin film 13-2electrically connecting the first electrical conductor 12-1 and thesecond electrical conductor 12-3 to each other is formed bymetallization. The first terminator 13-1 is comprised of a thin-filmresistor composed of Ta₂N, and has a sheet resistance of 100 ohms per aunit area and an impedance of ZL1 of 150 ohms.

Similarly, the second stripline 12 is comprised of a coplanar linecomprised of an alumina substrate 14-2 having a dielectric constant of9.95, a thickness of 250 micrometers, a length of 3.5 mm, and a width of700 micrometers, and has a characteristic impedance of 50 ohms. Each ofthe second thin film 15-2 and the third thin film 16-2 both electricallyconnecting the second electrical conductor 14-1 and the third electricalconductor 14-3 to each other is formed by metallization. The secondterminator 15-1 is comprised of a thin-film resistor composed of Ta₂N,and has a sheet resistance of 100 ohms per a unit area and an impedanceof ZL2 of 150 ohms. Similarly, the third terminator 16-1 is comprised ofa thin-film resistor composed of Ta₂N, and has a sheet resistance of 100ohms per a unit area and an impedance of ZL3 of 150 ohms.

The optical modulator 11, the first stripline 12 and the secondstripline 14 are arranged on the metal carrier 7 in a lineperpendicularly to an optical axis of the optical modulator 11.

The first stripline 12 and the optical modulator 11 are electricallyconnected to each other through the first Au ribbon wire 18 as a bondingwire having an inductance of 150 pH. Similarly, the second stripline 14and the optical modulator 11 are electrically connected to each otherthrough the second Au ribbon wire 19 as a bonding wire having aninductance of 180 pH.

The optical modulator 11 as a part of the electroabsorption type opticalmodulator module 20 is further explained hereinbelow.

FIG. 3 is a perspective view of the optical modulator 11.

The illustrated optical modulator 11 is an electrical-field absorptiontype optical modulator, and is comprised of a substrate 42, a firstelectrode 41 formed on a lower surface of the substrate 42, an opticalabsorption layer 43 formed almost centrally of the substrate 42 in alength-wise direction of the substrate 42, a first cladding layer 44formed on the optical absorption layer 43, a contact layer 45 formed onthe first cladding layer 44, a second electrode 46 formed on the contactlayer 45, second cladding layers 47 formed around the optical absorptionlayer 43 and the first cladding layer 44, and an electrically insulatinglayer 48 formed on the second cladding layers 47.

The substrate 42 is comprised of an InP substrate.

The second cladding layers 47 are comprised of an n-InP layer formed onthe substrate 42 in a region other than a region in which the opticalabsorption layer 43 is formed.

The optical absorption layer 43 is sandwiched between the secondcladding layers 47 on the substrate 42. The optical absorption layer 43has a width of 2 microns, and has an undoped InGaAs/InAlAs quantum wellstructure (the number of well layers Nw=7) having a wavelength of 1.49micrometers. The optical absorption layer 43 acts as an opticalwaveguide, and allows the optical signal 31 to pass therethrough. Theoptical absorption layer 43 optically modulates the optical signal 31,and outputs the optical signal 32.

The first cladding layer 44 is sandwiched between the second claddinglayers 47, and is comprised of a p-InP layer covering an upper surfaceof the optical absorption layer 43 therewith.

The contact layer 45 is sandwiched between the second cladding layers47, and is comprised of a p⁺-InGaAs layer covering an upper surface ofthe first cladding layer 44 therewith.

The second electrode 46 is sandwiched between the second cladding layers47, and is comprised of a p-electrode having a five-layered metalstructure, Cr/Au/Ti/Pt/Au, and covering an upper surface of the contactlayer 45 therewith. The second electrode 46 partially extends onto theelectrically insulating layer 48, and defines a connection 49 throughwhich the first and second bonding wires 18 and 19 are connected to eachother. The modulation RF signal 33 having been transmitted through thefirst stripline 12 is applied to the connection 49.

The electrically insulating layer 48 is formed covering surfaces of thesecond cladding layers 47 therewith.

The first electrode 41 is comprised of an n-electrode having athree-layered metal structure, Ti/Pt/Au.

The optical modulator 11 has an element length of 300 micrometers, andis formed on opposite cleavage surfaces 40 thereof with low-reflectionfilms (not illustrated) having an index of refraction of 0.1% orsmaller. The optical modulator 11 has an element capacity of 125 fF whena reverse-bias voltage of −2V is applied thereto.

FIG. 5 is a graph showing the results of measurement of a relation(frequency-response characteristic) between a frequency and a signalintensity of the modulation RF signal 33 in the circuit 27 illustratedin FIG. 4. The axis of abscissa indicates a frequency of the inputmodulation RF signal 33, and the axis of ordinates indicates anintensity of reflection or transmission. The solid line indicates themeasurement results obtained when the circuit 27 illustrated in FIG. 4is used, and the broken line indicates the measurement results obtainedwhen a conventional circuit for launching a modulation signal to anoptical modulator is used.

Applying a reverse-bias of −2V to the electroabsorption type opticalmodulator 11, the reflection and transmission reflecting the opticalmodulator 11 and a circuit for launching a modulation signal to theoptical modulator 11 were measured for the input modulation RF signal33. As shown with a curve (solid line) indicating the reflection S11, itwas found out that the reflection wave was suppressed down to a slightdegree, specifically, −15 dB or smaller, when the modulation RF signal33 had a frequency in the range of zero (direct current) and 60 GHz.

When a signal light having a wavelength of 1550 nm was input into theoptical modulator 11 of the circuit 27, a modulation frequency band(transmission S21: −3 dB or greater) became 50 GHz or greater. Thus, itwas found out that there was obtained a broad-band optical modulationcharacteristic practically sufficient for accomplishing 40 GHzoptical-fiber communication.

EXAMPLE 2

Hereinbelow is explained another example of applying the circuit 27 forlaunching a modulation signal to an optical modulator, illustrated inFIG. 4, to an electroabsorption type optical modulator module.

FIG. 2 is an exploded plan view of the electroabsorption type opticalmodulator module 20 including the circuit 27 for launching a modulationsignal to an optical modulator, illustrated in FIG. 4. Theelectroabsorption type optical modulator module 20 has the samestructure as that of the electroabsorption type optical modulator moduleexplained in Example 1. The carrier 7 is composed of Fe—Ni—Co alloyunlike the carrier 7 in Example 1.

Similarly to the Example 1, applying a reverse-bias of −2V to theelectroabsorption type optical modulator 11, the reflection of theoptical modulator 11 and a circuit for launching a modulation signal tothe optical modulator 11 were measured for the input modulation RFsignal 33. The reflection wave was suppressed down to a slight degree,specifically, −15 dB or smaller, when the modulation RF signal 33 had afrequency in the range of zero (direct current) and 40 GHz.

When a signal light having a wavelength of 1550 nm was input into theoptical modulator 11 of the circuit 27, a modulation frequency bandbecame 50 GHz or greater. Thus, it was found out that there was obtaineda broad-band optical modulation characteristic practically sufficientfor accomplishing 40 GHz optical-fiber communication.

When the optical modulator module 20 is reduced into practice, it isimportant to determine tolerances for parameters of circuit elements inorder to avoid variance in dimensions and resistances caused infabrication of the optical modulator module 20 from exerting harmfulinfluence on the modulation characteristics.

FIG. 6 is a graph showing the results of measurement of a relation(frequency-response characteristic) between a frequency and a signalintensity of the modulation RF signal 33 in the circuit 27 in Example 2.The axis of abscissa indicates a frequency of the input modulation RFsignal 33, and the axis of ordinates indicates an intensity ofreflection or transmission. The curves illustrated in FIG. 6 indicatereflection and transmission found when the characteristic impedance Z0of a line through which the modulation RF signal 33 is input varieswithin ±5%, when a sum of resistances of the terminators varies within±10%, and when a capacity Cabs of the optical modulator 11 varies within±25%.

As illustrated in FIG. 6, even when the characteristic impedance of eachof the striplines, the resistances of the terminators and the capacityof the optical modulator vary within ±5%, ±10% and ±25%, respectively,the reflection S11 of the circuit 27 for launching a modulation signalto an optical modulator is kept at −13 dB or smaller.

Furthermore, a modulation frequency band (transmission S21: −3 dB orgreater) is equal to or greater than 37 GHz. Thus, it was found out thatthere was obtained a practically sufficient broad-band andlow-reflection optical modulation characteristic.

As having been explained so far, the circuit for launching a modulationsignal to an optical modulator, an optical modulator module and a methodof launching a modulation signal to an optical modulator all inaccordance with the present invention provide the following advantages.

The first advantage is that it is possible to significantly reduce thereflection of a circuit for launching a modulation signal to an opticalmodulator, down to a practically ignorable level in a high-frequencyband close to a millimeter wave band, and to broaden a band of amodulation characteristic.

This is because the circuit for launching a modulation signal to anoptical modulator, including terminators in a dispersion manner, canindependently optimize upper and lower limits of an impedance of amatching circuit, by controlling dependency of behavior of the opticalmodulator as a capacitor on a frequency.

The second advantage is that it is possible to reduce a burden acting ona driver circuit for driving an optical modulator and an opticalmodulator module including an optical device in which optical modulatorsare integrated, ensuring that the driver circuit can be fabricated tohave a broader band, fabricated to be in a smaller size, fabricated tooperate at a smaller voltage, and fabricated in smaller costs.

The reason is as follows. Since it is possible to significantly reducethe reflection of a circuit for launching a modulation signal to anoptical modulator, down to a practically ignorable level in a broadrange, as mentioned in the first advantage, it would be possible todrive an optical modulator module at a smaller voltage (current),resulting in that a burden acting on the circuit elements can bereduced, designability of selecting elements and fabricating a circuitfor broadening a band is enhanced, and power consumption can be reduced.

The third advantage is that it is possible to fabricate in smaller costsan optical modulator and an optical modulator module including anoptical device in which optical modulators are integrated.

The reason is as follows. Since the circuit for launching a modulationsignal to an optical modulator in accordance with the present inventioncan be fabricated merely by applying minimum change, that is, additionof thin-film resistors constituting terminators, to striplines in aconventional circuit for launching a modulation signal to an opticalmodulator, it would be possible to use an existing equipment forfabricating the circuit, and a conventional process for doing the same,though a mask pattern for fabricating thin-film resistors is necessaryto be partially revised. Thus, further costs for accomplishing theabove-mentioned first and second advantages can be avoided.

In a conventional high-speed optical modulator used for an optical-fibercommunication system, and a conventional optical modulator moduleincluding an optical device in which optical modulators are integrated,when they are driven, the significant increase in reflection in ahigh-frequency band was a serious problem. The circuit for launching amodulation signal to an optical modulator in accordance with the presentinvention solves the serious problem by applying a function of matchingwaves of a signal light to end surfaces of an optical waveguide circuitduring fabrication of the optical waveguide circuit, without using anyadditional optical parts. Thus, a hybrid integrated optical modulatormodule can be fabricated in a smaller size, with higher performances,and at a smaller cost.

While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by way of the present invention is not to be limitedto those specific embodiments. On the contrary, it is intended for thesubject matter of the invention to include all alternatives,modifications and equivalents as can be included within the spirit andscope of the following claims.

The entire disclosure of Japanese Patent Application No. 2002-066620filed on Mar. 12, 2002 including specification, claims, drawings andsummary is incorporated herein by reference in its entirety.

1. A circuit for launching a modulation signal to an optical modulator,comprising: an optical modulator for modulating an optical signal; afirst stripline electrically connected to said optical modulator anddirecting a modulation RF signal to said optical modulator; and a secondstripline electrically connected to said first stripline through saidoptical modulator, wherein said first stripline includes a firstelectrical device and has a first characteristic impedance, said secondstripline includes a second electrical device and has a secondcharacteristic impedance, said first characteristic impedance is equalto a characteristic impedance of a path through which said modulation RFsignal is input into said first stripline, and a parallel-combinedimpedance of said first and second electrical devices is equal to saidcharacteristic impedance of said path.
 2. The circuit as set forth inclaim 1, wherein said second electrical device is comprised of at leasttwo electrical devices, which are arranged at different locations fromeach other in a length-wise direction of said second stripline.
 3. Thecircuit as set forth in claim 2, wherein said at least two electricaldevices are arranged at opposite ends of said second stripline.
 4. Thecircuit as set forth in claim 1, wherein at least on of said first andsecond electrical devices is comprised of a resistor.
 5. The circuit asset forth in claim 4, wherein said resistor is comprised of a thin-filmresistor formed on an electrical conductor of said second stripline. 6.The circuit as set forth in claim 1, wherein said second stripline hasan electrical length equal to or smaller than a quarter of a wavelengthassociated with a maximum frequency of said modulation RF signal.
 7. Thecircuit as set forth in claim 1, wherein said second characteristicimpedance is different from said first characteristic impedance.
 8. Thecircuit as set forth in claim 1, wherein said first electrical devicehas an impedance different from said first characteristic impedance. 9.An optical modulator module comprising: a high-frequency input sectionfor receiving a modulation RF signal by which an optical signal ismodulated; an optical input section for receiving a first opticalsignal; a circuit for launching a modulation signal to an opticalmodulator, electrically connected to said high-frequency input sectionand optically connected to said optical input section, said circuitmodulating said first optical signal into a second optical signal inaccordance with said modulation RF signal, said circuit being comprisedof a circuit defined in any one of claims 1 to 8, and an optical outputsection optically connected to said circuit and outputting said secondoptical signal.
 10. The optical modulator module as set forth in claim9, wherein said optical input section is comprised of: an optical inputterminal through which an optical signal is input and output; and afirst lens, and wherein said optical input terminal is connected to afirst optical fiber, receives said first optical signal through saidfirst optical fiber, and outputs said first optical signal to said firstlens, and said first lens receives said first optical signal and outputssaid first optical signal to said optical modulator of said circuit. 11.The optical modulator module as set forth in claim 9, wherein saidoptical output section is comprised of: a second lens; and an opticaloutput terminal through which an optical signal is input and output, andwherein said second lens receives said second optical signal from saidoptical modulator of said circuit, and outputs said second opticalsignal to said optical output terminal, and said optical output terminalis connected to a second optical fiber, receives said second opticalsignal through said second lens, and outputs said second optical signalto said second optical fiber.
 12. A method of launching a modulationsignal to an optical modulator, comprising: outputting a modulation RFsignal to an optical modulator through a first stripline including afirst electrical device and having a first characteristic impedance;modulating a first optical signal into a second optical signal in saidoptical modulator in accordance with said modulation RF signal; andoutputting said modulation RF signal through said optical modulator to asecond stripline including a second electrical device and having asecond characteristic impedance, wherein said first characteristicimpedance is equal to a characteristic impedance of a path through whichsaid modulation RF signal is input into said first stripline, and aparallel-combined impedance of said first and second electrical devicesis equal to said characteristic impedance of said path.